Broad Frequency Spectrum Research Articles

Dual Electro-Optic Comb Generator by Heterodyning Two Combs for Extending the Optical Bandwidth

A novel dual-electro-optic (DEO) comb generator is proposed by heterodyning the optical frequency combs (OFCs) to achieve a high bandwidth of ~2.74 THz with an efficient and cost-effective structure. This can attract the research community’s interest, particularly for the inherent support in optical communications due to its simplicity and high throughput. The OFC is generated by cascading a continuous wave (CW) laser and Mach Zehnder modulator (MZM) coupled to a second laser source that produces a broad spectrum of frequencies by the frequency heterodyning after passing it through a second MZM. This is the highest number of comb lines produced so far in cascaded configurations of the electro-optic modulators. The frequency difference between the two lasers is carefully selected, equal to 1.38 THz, which combinedly produces around 137 well-flattened OFC lines with 20 GHz frequency spacing and more than 40 dBm tone to noise ratio and an amplitude difference of 0 to 6 dB. The bandwidth of the generated flattened comb source is approximately 2.74 THz, which covers over half of the optical C-band.

Investigation of structural, topological, and electrical properties of scheelite strontium molybdate for electronic devices

This article describes the synthesis (solid-state reaction) and characterizations of a scheelite strontium molybdate. The consistency of lattice parameters by N-TREOR09 and the Le-Bail Rietveld refinement method supports the tetragonal crystal system. The spectrum of the pair distribution function (PDF) depicts the orientations of atoms within a tetragonal symmetry. The Williamson-Hall plot examines the average crystallite size (86 nm) and micro-lattice strain (0.00091). The scanning electron microscopic image, energy dispersive X-ray (EDX) spectra, and elemental mapping confirm the uniform and isotropic characteristics of the synthesized material, purity and compactness, and topographic roughness according to ISO25178 standards. The investigation of dielectric, impedance, and conductivity spectra confirms the presence of Maxwell-Wagner dielectric dispersion, relaxation, and influential transport phenomena across a broad spectrum of experimental frequencies (ranging from 1 kHz to 1 MHz) and temperatures (from 25 °C to 500 °C). The observed temperature-dependent resistance characteristics validate that the prepared material is well-suited for applications involving PTC thermistors, sensors, and other related devices. The scaling properties of the modulus data provide evidence for a non-Debye relaxation mechanism. The values of the varistor constant, β1, β2, and β3, which are 1.414, 2.362, and 9.654, respectively, indicate the existence of Ohmic conduction, trap-limited space charge limited conduction (SCLC), and trap-filled SCLC mechanisms suitable for electronic devices.

A multilayer triboelectric-electromagnetic hybrid nanogenerator for vibration energy harvesting and frequency monitoring

With the advancement of sensor networks, the exploration of distributed vibrational energy resources bears significant implications. We introduce a multilayer triboelectric-electromagnetic hybrid generator (M-TEHG). The M-TEHG exhibits remarkable output capabilities across a broad frequency spectrum, with a peak power density of 2.8 W/m3 at 10 Hz. Moreover, by incorporating the gas discharge tube (GDT) as a rapid switch in the BUCK circuit, the M-TEHG provides a continuous and stable power supply for the load. This innovation not only enhances the charging rate of the M-TEHG's capacitor by 43.84–57.66%, but also enables uninterrupted power supply to temperature and humidity sensors. Additionally, the M-TEHG also possesses the capability to monitor vibration frequencies. It utilizes a linear bearing and smooth guide rod structure, effectively mitigating the impact of vibration signals from different angles. The frequency monitoring of the M-TEHG achieves an accuracy of up to 99.1% and demonstrates exceptional linearity. The integrated electricity generation and sensing capability of M-TEHG demonstrate its immense potential in various fields, such as distributed sensor networks, unmanned environment monitoring, and especially early warning systems for bridge vibration.

Comparison of Heat Sinks in Back-End of Line to reduce Self-Heating in 22FDX® MOSFETs

This work compares the effect of different heat sink dimensions in the back-end of line (BEOL) on the self-heating (SH) parameters. To assess the self-heating (SH) parameters, the RF characterization technique is employed, which involves measuring S-parameters across a broad frequency spectrum. In addition to the standard device without a sink, two sinks of different dimensions, both connected to the gate, are considered: one smaller and one bigger in size. We experimentally demonstrate lower self-heating as the sink size increases. By using a smaller and a larger sink, a mean reduction of 4.7% and 8.9% in thermal resistance is observed, showing greater SH reduction with increasing sink size, which is also verified from comparison with previous works. The impact of the sinks on RF figures-of-merit (FoMs), namely the cutoff frequency fT and maximum oscillation frequency fmax is investigated. fT is seen to degrade stronger due to increased gate capacitance whereas fmax degradation is limited because of reduced gate resistance.

Direct numerical simulation of slender cones with variable nose bluntness based on graphics processing unit computation

Direct numerical simulation with up to 10×109 scale grid points based on graphics processing unit computation is carried out to investigate the bluntness effect on the hypersonic boundary-layer transition over a slender cone with zero angle of attack at Mach 6. Four cases with the nose radii of 1, 10, 20, and 40 mm are conducted, and the corresponding Reynolds number based on the nose radius varies from 1.0×104 to 4.0×105. Random disturbances with a broad spectrum of frequencies and a wide range of azimuthal wavenumbers were applied to the wall to simulate disturbances caused by wall roughness. The numerical results show that as the nose tip radius increases, the transition position gradually moves downstream with increased transition region. For the case with a nose radius of 1 mm, the flow transition and entropy swallowing occur almost simultaneously, while for other cases, the transition takes place earlier than the entropy swallowing. In consequence, the disturbance amplitude upstream of the transition in the 1 mm case is much larger than that of other cases. To further study the mechanism of the transition, the frequency spectrum analysis is carried out. It is found that all cases exhibit two characteristic frequencies within the transition region, i.e., the high frequency and extremely low frequency. Owing to the influence of the entropy layer, the characteristic high frequency of the 1 mm case is significantly higher than that of other cases. With the increase in the nose radius, the characteristic frequency of the high frequency decreases gradually.

Simultaneous Terahertz Pulse Generation and Manipulation with Spintronic Coding Surface

AbstractThe utilization of spintronic terahertz (THz) emitters has a significant influence on broadband coherent spectroscopy and imaging at THz frequencies. By using femtosecond light to irradiate the spintronic heterostructures, spin currents from the ultrafast demagnetization effect can be transformed into charge currents through the inverse spin Hall effect, leading to the emission of THz waves. In this work, through the incorporation of sequential and inverse deposition of W‐Co20Fe60B20‐Pt heterostructure films, control is gained over the spin currents and THz waveforms, enabling it to achieve a 180° phase difference across a broad frequency spectrum while efficiently radiating THz waves. This, in turn, allows it to control the spatial phase of THz radiation. To test the device's capabilities, lithography‐based devices capable of focusing, producing vortexes, and dividing beams are manufactured. This investigation aims to advance the development of THz pulse sources while providing essential support for cutting‐edge technologies such as THz spectroscopy, high‐resolution imaging, and high‐speed communications.

A Compressible, Coupled Three-Dimensional/One-Dimensional Flow Solution Method for the Seamless Simulation of Centrifugal Pumps and Their Piping

Abstract A compressible finite volume Navier–Stokes flow solver is coupled to a method of characteristics for the seamless turbulent flow simulation of entire pump systems. For the pump, three-dimensional (3D) simulations including cavitating flow conditions are performed, and the piping is treated one-dimensional (1D) by a method of characteristics. Thus, classical boundary conditions at the suction and pressure pipe of the 3D computational domain of the pump are substituted by a two-way coupled 1D piping simulation method. Particular emphasis has been placed on the non-reflecting properties of the 3D–1D coupling interface. For validation, in-house experiments are performed on a low specific speed centrifugal pump in a closed-loop facility. For cavitating flow conditions, excitation on the pump's pressure side by rotor–stator interaction is enhanced over a broad frequency spectrum due to collapsing voids. The suction side piping is shielded by void regions within the blading from the excitation on the pump's pressure side, leading to an acoustic decoupling of the suction side. These experimental observations are reproduced by the new seamless simulation method. In particular, the measured pressure amplitudes are well reproduced for a broad frequency spectrum, at several piping positions, and for a variation of the flow rate and the cavitation intensity. Remaining deviations to experimental data are traced back to the omission of structural compliance and uncertainties regarding the pressure side piping modeling, which will be addressed in future studies.

Finite-momentum instability of a dynamical axion insulator

Due to the chiral anomaly, Weyl semimetals can exhibit a signature topological magnetoelectric response known as an axion term which is determined by the microscopic band structure. In the presence of strong interactions Weyl fermions may form a chiral condensate, with the intrinsic dynamics and fluctuations of the associated condensate phase producing a dynamical contribution to the axion response. Here we show that an imbalance in the density of right- and left-handed electrons drives an instability of the chiral condensate towards finite momentum and leads to strong fluctuations in the axion response. We derive a long-wavelength theory of Lifschitz type governing the dynamics of the Goldstone mode and use this to characterize its associated spatial fluctuations, which manifest as an inhomogeneous anomalous Hall effect. We show that these fluctuations produce signatures in inelastic light-scattering experiments across a broad spectrum of frequencies, and can be used to determine the structure factor for the axionic collective mode.

Backward masking reveals coarse-to-fine dynamics in human V1

Natural images exhibit luminance variations aligned across a broad spectrum of spatial frequencies (SFs). It has been proposed that, at early stages of processing, the coarse signals carried by the low SF (LSF) of the visual input are sent rapidly from primary visual cortex (V1) to ventral, dorsal and frontal regions to form a coarse representation of the input, which is later sent back to V1 to guide the processing of fine-grained high SFs (i.e., HSF). We used functional resonance imaging (fMRI) to investigate the role of human V1 in the coarse-to-fine integration of visual input. We disrupted the processing of the coarse and fine content of full-spectrum human face stimuli via backward masking of selective SF ranges (LSFs: <1.75cpd and HSFs: >1.75cpd) at specific times (50, 83, 100 or 150 ms). In line with coarse-to-fine proposals, we found that (1) the selective masking of stimulus LSF disrupted V1 activity in the earliest time window, and progressively decreased in influence, while (2) an opposite trend was observed for the masking of stimulus’ HSF. This pattern of activity was found in V1, as well as in ventral (i.e. the Fusiform Face area, FFA), dorsal and orbitofrontal regions. We additionally presented subjects with contrast negated stimuli. While contrast negation significantly reduced response amplitudes in the FFA, as well as coupling between FFA and V1, coarse-to-fine dynamics were not affected by this manipulation. The fact that V1 response dynamics to strictly identical stimulus sets differed depending on the masked scale adds to growing evidence that V1 role goes beyond the early and quasi-passive transmission of visual information to the rest of the brain. It instead indicates that V1 may yield a ‘spatially registered common forum’ or ‘blackboard’ that integrates top-down inferences with incoming visual signals through its recurrent interaction with high-level regions located in the inferotemporal, dorsal and frontal regions.

Electrical Impedance Spectroscopy for Ex-Vivo Breast Cancer Tissues Analysis.

This paper describes a cancer detection procedure based on customised electrical impedance spectroscopy (EIS) in breast cancer surgical samples and an analysis of its outcomes. A tissue analyser was developed to inject a low-amplitude alternating current with penetrating electrodes into breast specimens along a broad spectrum of frequencies. Experimental measurements were carried out on more than one hundred excised breast cancer specimens, with the goal of discriminating between the tumour and surrounding non-neoplastic tissue. The probe was inserted in different locations immediately after surgical excision in order to measure tissue impedance (modulus and phase). Electrical impedance varied significantly between neoplastic and surrounding non-neoplastic tissues, with a low standard deviation among the different measurements, confirming good reproducibility. Tumours could be discriminated from non-neoplastic tissues according to their impedance modulus value for high frequencies and phase value for low frequencies. Impedance also varied significantly in both non-neoplastic and tumour tissues depending on the patient's age and tumour characteristics, such as size and histological sub-type. EIS is able to discriminate between healthy tissue and cancer. Future developments of this technology could be exploited for intraoperative real-time evaluation of the transition zone between cancer and normal tissues.

Dynamics of the pedestal in the recovery phase in EAST type-I ELM plasmas

In the Experimental Advanced Superconducting Tokamak high-confinement mode plasmas, a low-frequency n = 1 magnetic coherent mode with frequency f = 20–50 kHz and a high-frequency mode with electromagnetic characteristics (HFEM, f ∼ 280 kHz) have been found between type-I edge localized modes (ELMs). Both the HFEM and the n = 1 mode are located in the pedestal region, but their radial locations seem to be somewhat different. It seems from the present data that the HFEM is closer to the maximum density gradient region, while the n = 1 mode may be closer to the separatrix. The experimental results demonstrate that the electron temperature recovers more rapidly than the pedestal density, and the n = 1 mode is excited in the pedestal after an ELM collapse. With the increase in the pedestal density, the HFEM appears and becomes dominant, while the amplitude of the n = 1 mode decreases significantly. The observations indicate that the HFEM may suppress the amplitude of the n = 1 mode. In the pre-ELM phase, the pedestal electron density and temperature are saturated, the characteristics of the HFEM show a significant change (a much broader frequency spectrum and reduced mode amplitude), and the n = 1 mode recovers again. Analysis using a wavelet bispectrum reveals that a nonlinear coupling between the n = 1 mode and the high-frequency magnetic fluctuations exists in the pre-ELM phase. The relations between the nonlinear mode coupling, the reappearance of the n = 1 mode and the ELM crash are discussed.

Shorted Micro‐Waveguide Array for High Optical Transparency and Superior Electromagnetic Shielding in Ultra‐Wideband Frequency Spectrum

AbstractWhile functional materials with both light transmitting and electromagnetic shielding are highly desirable and have made rapid advancements, only very few of them meet the stringent electromagnetic interference (EMI) shielding criteria for optoelectronic systems. Achieving high optical transparency and superior EMI shielding in a broad frequency spectrum is a remaining challenge in both academic and industrial areas. Herein, a design strategy of shorted micro‐waveguides (SMWs) array to decouple the light transmission and EMI shielding is proposed and experimentally demonstrated. The array of SMWs, consisting of cutoff metallic micro‐waveguides and shorting indium tin oxide (ITO) continuous conductive film, exhibits high optical transmittance of 90.4% and superior EMI shielding effectiveness of 60.8 dB on average over ultra‐wide frequency spectrum (0.2–1.3 GHz &amp; 1.7–18 GHz). Compared to previously reported works, an improvement of 17 dB in average shielding effectiveness is achieved under the same level of light transmission, and the shielding frequency spectrum is significantly expanded. The working principle is explained in depth and factors influencing the performance are investigated for design optimization. These outstanding properties enable the transparent shielding material based on SMWs to excel in future applications of EMI shielding for optoelectronic systems.

Inference of the Timescale‐Dependent Apparent Viscosity Structure in the Upper Mantle Beneath Greenland

AbstractContemporary crustal uplift and relative sea level (RSL) change in Greenland is caused by the response of the solid Earth to ongoing and historical ice mass change. Glacial isostatic adjustment (GIA) models, which seek to match patterns of land surface displacement and RSL change, typically employ a linear Maxwell viscoelastic model for the Earth's mantle. In Greenland, however, upper mantle viscosities inferred from ice load changes and other geophysical phenomena occurring over a range of timescales vary by up to two orders of magnitude. Here, we use full‐spectrum rheological models to examine the influence of transient deformation within the Greenland upper mantle, which may account for these differing viscosity estimates. We use observations of shear wave velocity combined with constitutive rheological models to self‐consistently calculate mechanical properties including the apparent upper mantle viscosity and lithosphere thickness across a broad spectrum of frequencies. We find that the contribution of transient behavior is most significant over loading timescales of 102–103 years, which corresponds to the timeframe of ice mass loss over recent centuries. Predicted apparent lithosphere thicknesses are also in good agreement with inferences made across seismic, GIA, and flexural timescales. Our results indicate that full‐spectrum constitutive models that more fully capture broadband mantle relaxation provide a means of reconciling seemingly contradictory estimates of Greenland's upper mantle viscosity and lithosphere thickness made from observations spanning a range of timescales.

Exponential data encoding for quantum supervised learning

Reliable quantum supervised learning of a multivariate function mapping depends on the expressivity of the corresponding quantum circuit and measurement resources. We introduce exponential-data-encoding strategies that are hardware-efficient and optimal among all nonentangling Pauli-encoded schemes, which is sufficient for a quantum circuit to express general functions having very broad Fourier frequency spectra using only exponentially few encoding gates. We show that such an encoding strategy not only reduces the quantum resources, but also exhibits practical resource advantage during training in contrast with known efficient classical strategies when polynomial-depth training circuits are also employed. When computation resources are constrained, we numerically demonstrate that even exponential-data-encoding circuits with single-layer training modules can generally express functions that lie outside the classically expressible region, thereby supporting the practical benefits of such a resource advantage. Finally, we illustrate the performance of exponential encoding in learning the potential-energy surface of the ethanol molecule and California's housing prices.

Single-Cycle Terahertz Pulse Generation via Superimposed Diffraction With Cherenkov Effect

The ultrashort terahertz (THz) pulse with a broad frequency spectrum has important applications in THz spectroscopy and imaging. Previous THz-pulse generating methods are restricted by either low power or high cost. Here, we propose a novel scheme for generating ultrashort THz pulses, which uses a low-energy sheet electron beam to drive an inclined array of size-graded grooves. The diffraction radiation (DR) from each groove is successively generated and coherently superimposed at a specified direction via the Cherenkov effect. A single-cycle THz pulse with a spectrum band covering the region from 0.1 to 1 THz can be realized. In addition, using several arrays of grooves, a train of ultrashort THz pulses can be further obtained. The radiation power, which depends on the beam current, reaches as high as several kilowatts by using available electron beams. It affords an effective way of generating ultrashort THz pulses with broad application prospects.

Ultrasonic Measurement of Axial Preload in High-Frequency Nickel-Based Superalloy Smart Bolt

A high-frequency, piezoelectric thin-film sensor was successfully deposited on a nickel-based superalloy bolt by radio frequency magnetron sputtering to develop a smart, nickel-based superalloy bolt. Ultrasonic response characterization, high accuracy, and repeatability of ultrasonic measurement of axial preload in nickel-based superalloy smart bolts are reported here and were fully demonstrated. The axial preload in the nickel-based superalloy smart bolt was directly measured by the bi-wave method (TOF ratio between transverse and longitudinal-mode waves) without using the traditional integration of a longitudinal and shear transducer. A model concerning the bolt before and after tensioning was established to demonstrate the propagation and displacement distribution of the ultrasonic waves inside a nickel-based superalloy smart bolt. The measured A-scan signal presented significantly favorable features including a mixture of transverse and longitudinal mode waves, a pure and broad frequency spectrum which peaked at 17.14 MHz, and high measurement accuracy below 3% for tension of 4 kN-20 kN. For the temporal ultrasonic signal, the measurement envelopes were narrower than for the counterpart of the simulation, justifying the 'filtration' advantage of the high-frequency sensor. Both the TOF change of the single longitudinal-mode wave and the TOF ratio between transverse- and longitudinal-mode waves increased linearly with preload force in the range of 0 kN to 20 kN. Compared with the commercial piezoelectric probe, the proposed probe, based on the combination of a high-frequency, piezoelectric thin-film sensor and a magnetically mounted transducer connector, exhibited high tolerance to temperatures as high as 320 °C and high repeatability free from some interference factors such as bolt detection position change and couplant layer thickness. The results indicate that this system is a promising axial preload measurement system for high-temperature fasteners and connectors, and the proposed sensor is a practical, high-frequency ultrasonic sensor for non-destructive testing.

Modeling Ocean Wave Transfer to Ross Ice Shelf Flexure

AbstractAn efficient mathematical model is presented for predicting the transfer of ocean waves to ice shelf flexure along two‐dimensional transects. The model incorporates varying ice shelf thickness and seabed bathymetry profiles, and is able to predict responses of large ice shelves over a broad frequency spectrum. The model is used to generate displacement and strain transfer functions for the Ross Ice Shelf (RIS) using geometries from the Bedmap2 data set. The transfer functions are validated against recent observations and used to study the influence of geometrical variations on strain transfer close to the shelf front. Predictions of RIS strain in response to example irregular incident swell and infragravity waves are generated over a wide region, and show similar maximum strains but contrasting spatial strain patterns. The model and results provide a basis for studying destabilizing impacts of ocean waves on the RIS.

Furious Feedback and the Revolutionary Ode to Noise

Furious Feedback and the Revolutionary Ode to Noise

Metagrating absorber: design and implementation.

The extent to which the introduction of subwavelength spatial modulation of electromagnetic properties improves absorption performances is studied. The proposed absorber represents an evolution from the Salisbury screen, whereby the uniform resistive layer is replaced by a metagrating. A periodic supercell that supports only the specular reflection is first designed, and load impedances are then engineered to suppress this diffraction mode. To experimentally demonstrate the concept, four prototypes are fabricated and tested in the microwave domain around 10 GHz. Furthermore, the performances assessed by a merit factor derived from Rozanov's bound show that the use of metagratings opens up good perspectives for improving the state of the art. Our findings can pave the way toward the development of high performance absorbers for applications across a broad frequency spectrum.

Delamination Detection in Concrete Plates using a Laser Ablation Impact Echo Technique

This study investigates the detection of delaminations in concrete plates using non-contact laser ablation instead of the conventional hammer excitation. Tests were performed on two specimens of which one had an artificial delamination. 5 mm to 15 mm diameter steel tip hammers were used as a reference source and compared to a 7 ns pulsed 1064 nm Nd:YAG laser with 150 mJ pulse energy. Signals were recorded by surface- mounted accelerometers and in air by microphones and a laser Doppler vibrometer. Frequencies up to 150 kHz are excited with a broad frequency spectrum and little energy compared to the narrower frequency spectra of the hammers. The characteristic thickness resonance frequency of the intact plates can be accurately detected by the hammer excitation. The laser can excite low frequency flexural vibration modes over a shallow delamination at 3 cm depth, but does not excite the thickness resonance frequency. The low frequency flexural vibrations are verified by numerical natural frequency analysis.